MICROSTRUCTURAL INSIGHTS INTO STRAIN VARIATION ALONG THE MAIN CENTRAL THRUST SYSTEM; EASTERN NEPAL HIMALAYA

The location and style of deformation along the Main Central Thrust (MCT) in the Nepalese Himalaya is a topic of considerable debate. Current tectonic models proposed for the Himalaya view the MCT as either: 1) a migrating distributed zone of deformation accommodating top-to-the-south-directed translation of the mid-crust or; 2) a discrete series of foreland propagating, southward verging thrust faults. To provide a thorough characterization of the mechanisms by which strain is recorded along the MCT we have collected detailed structural data from four transects across the MCT zone in east-central Nepal. Field data and macrostructural analyses of folds, faults, and shear fabrics indicate that in this region, the MCT consists of a 2-4 km wide zone that separates pervasively ductily deformed, amphibolite-grade metamorphic rocks in its hanging wall from brittley deformed, greenschist-grade metapelites in its footwall. Microstructural data record evidence of three successive phases of progressively lower temperature deformation that affected units in the immediate MCT hanging wall. Moreover, electron backscatter diffraction data from quartzites and quartz-rich schists show that in the hanging wall of the MCT deformation is dominated by simple shear with a minor component of pure shear. In contrast, deformation of rocks in the footwall of the MCT zone occurred by simple shear alone. Deformation temperatures estimated from opening angles of cross-girdled quartz c axes fabrics are in agreement with those indicated by microstructural deformation regimes and show a temperature decrease of ~250˚C down structural section across the MCT zone, a trend that is consistent with those observed in studies of the MCT in northwest India. Combined, these data indicate that, in eastern Nepal, the MCT is best described as a diffuse (2-4 km wide) zone of high strain rather than a discrete fault(s) that juxtaposes units with very different structural and metamorphic histories.